ci: autolock closed issues and PRs after 30 days (#6398)

Old issues and PRs occasionally get comments well after they have been
closed. These comments can lead to community frustration, as closed
issues and PRs aren't actively monitored.

This commit introduces a new CI job to automatically lock closed issues
and PRs after 30 days of inactivity. A `frozen-due-to-age` label is
added to denote why the issue has been locked.

This CI job is deliberately configured to silently close issues without
adding a comment to avoid notification noise for maintainers; adding a
label takes the place of a comment.

.github/workflows/autolock.yml

@@ -0,0 +1,25 @@
+name: Lock closed issues and PRs
+on:
+  workflow_dispatch: {}
+  schedule:
+    - cron: '0 0 * * *'
+
+permissions:
+  issues: write
+  pull-requests: write
+  discussions: write
+
+concurrency:
+  group: lock-threads
+
+jobs:
+  action:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/lock-threads@v5
+        with:
+          pr-inactive-days: 30
+          issue-inactive-days: 30
+          add-issue-labels: 'frozen-due-to-age'
+          add-pr-labels: 'frozen-due-to-age'
+          process-only: 'issues, prs'